gay蜜 is not a formal word, usually used on Internet or in oral languages by the young.
When a girl has a gay male close friend, she will call him "gay蜜".
e.g. "他是我的gay蜜。" means he's my close friend and he's gay.
Is "gay蜜""闺蜜" usually used for young adults (18-30 years old)? Yes.